Coextrusion of multiple layers of different polymers through a common die is used to produce a variety of polymer products, such as sheet, film, coatings, profiles, pipe, tubing, and foam-core products. Food and beverage packaging is a common application of such coextruded products. Sheet or film, for example, are often shaped after coextrusion into various packaging items by methods such as thermoforming, solid-phase pressure forming, or stamping. Preparation of tubing and hoses for use in automobiles and industrial equipment is another application. Individual polymers exhibit a wide variety of properties, such as permeability to oxygen, water vapor, and other gases and liquids, and chemical resistance to acids, bases, hydrocarbons, oils, alcohols, and other materials. Coextrusion of multiple polymer layers can be used to prepare a product with optimum properties for a particular application.
No comprehensive theory exists for predicting inter-layer adhesion in coextrusion of different polymers. Most knowledge comes from trial-and-error testing of the polymers. Even materials which form compatible blends do not necessarily exhibit good adhesion when coextruded. For example, a polyketone polymer and an ethylene vinyl alcohol copolymer will form a compatible blend, but will not adhere when coextruded. Adhesive polymers are commonly used as "tie layers" to bond incompatible polymers that do not normally adhere to each other. Ethylene-vinyl acetate, ethylene-acrylic acid, and ethylene-methyl acrylate copolymers are commonly used as tie layers.
Coextrusion of polymers without the use of tie layers allows the use of simpler equipment, and production of a lower cost product. It is an object of this invention to provide a coextruded product of two or more dissimilar polymer layers, produced without the use of any tie layers.
In an analogous processing technique, injection molding may be used to form composite parts by injecting two or more polymers into the same mold to make a composite part. The specialized techniques used to produce such composite parts include coinjection, dual shot, multi-shot, and shuttle molding. As an alternative, a pre-molded article may be inserted into a larger mold cavity, and a second polymer injected into the larger mold, coating part or all of the surface of the premolded article. This technique is known as insert molding. In the fabrication of such composite parts, it is essential to have strong adhesion between the different polymer surfaces to maintain the integrity of the product molded article. It is another object of this invention to provide a composite part, prepared through injection molding of two or more dissimilar polymers and without the use of any adhesive or tie layer between the dissimilar polymers.
In particular, it is an object of this invention to provide a coextruded product or composite part of a polyketone polymer and an other thermoplastic polymer, produced without the use of any adhesive or tie layer. Surprisingly, after extensive trial-and-error testing, only a few other thermoplastic polymers have been found that meet this objective.